This invention relates to a pneumatic power tool for tightening screw joints. Particularly, the invention concerns a screw joint tightening tool of the type including a housing, a primary motor for obtaining an initial degree of tightness in the joint, a secondary motor for obtaining the desired final degree of tightness in the joint and a power train for transferring the power of the motors to an output spindle connectable to the joint.
A tool of this type is disclosed in U.S. Pat. No. 3,529,513.
A problem concerned with previous power tools of this type is high air consumption. Of the dual motor nut running tools available today, both motors are simultaneously supplied with pressure air during the entire tightening process. This means that the secondary motor which always operates at a lower gear than the primary motor to accomplish the final degree of tightness in the joint runs unloaded during the initial stage of the process. By such an idle running of the secondary motor there is a considerable waste of pressure air, and, in addition to an undesirably high energy cost. Switching over to this fast working type of tool from the more common single motor tool of impacting or stalling type may give the result that very expensive measures have to be taken to up-size the compressor unit and the air distribution net.
In a further aspect of the high level of air consumption from which today's dual motor tools suffer it would be very difficult to adapt these tools to the specifications of a portable tool. Firstly, the required air supply and exhaust conduits can not be flexible and light enough to enable a comfortable handling of the tool, and, secondly, the air passages inside the tool housing, including a throttle valve, would have to be of such dimensions that the outer dimensions and weight of the tool housing would be unacceptable for a portable tool.